The preparation of dimeric cyclic esters of alpha-hydroxycarboxylic acids is an old and much studied process. The preparation is normally conducted in two stages involving first preparing an oligomer of the hydroxycarboxylic acid, i.e., a relatively short-chain condensation polymer thereof, then heating the oligomer to depolymerize it to the desired cyclic ester. The preparation of dimeric cyclic esters is discussed in Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); Bellis U.S. Pat. No. 4,727,163 (1988); Muller, Ger. Pat. Application Publication Nos. 3632103 and 3708915 (1988). In the preparation of the oligomers from the corresponding alpha-hydroxycarboxylic acids the water of condensation is difficult to completely remove from the polymer. Water is also formed in the depolymerization step so that the cyclic ester depolymerization product generally contains water as an impurity. The cyclic ester may also contain one or more open-chain hydroxycarboxylic acids as impurities. All such hydroxylic impurities are undesirable as they act as chain-stoppers in the subsequent polymerization of the cyclic ester to the high molecular weight products required for biomedical and other uses. It is therefore desired to keep the water and open-chain hydroxycarboxylic acid content of the dimeric cyclic ester as low as practicable.
U.S. Pat. No. 4,835,293, to Bhatia discloses an improved depolymerization and product recovery process for the production of dimeric cyclic esters such as lactide wherein a stream of an inert gas is employed to strip the cyclic ester from the reaction zone along with any water and/or volatile hydroxycarboxylic acid also formed therein. The resulting gaseous product stream is scrubbed with a polar organic solvent to recover the cyclic ester. The solvents include alcohols, ethers, esters and ketones, with use of isopropyl alcohol exemplifying the recovery of glycolide from its impurities. Isopropyl alcohol as scrubbing solvent solubilizes the hydroxycarboxylic acids and any water present, thereby enabling the recovery of glycolide directly from the scrubbing medium as a substantially insoluble filterable crystalline solid.
Use of an alcohol, however, as the scrubbing solvent for the recovery of glycolide, lactide or other such cyclic ester from a vapor product stream is not entirely satisfactory. It as well as water can react in the alcoholic solution to form open-chain products, which not only constitute a yield loss but further tend to increase the solubility of the cyclic ester in the scrubbing solution, further aggravating the yield loss problem.
On the other hand, use of a non-hydroxylic polar scrubbing solvent such as acetone, for example, which is non-reactive towards dimeric cyclic esters and in which the esters are highly soluble, likewise presents difficulties inasmuch as such polar solvent solubilizes the by-product hydroxycarboxylic acids as well, so that further processing would be required to separate the cyclic ester from the acids.
Water as a scrubbing solvent is also unsatisfactory in that heat transfer to it is much faster then mass transfer; consequently, the cyclic ester precipitates as a fog of particles, difficult to capture in the absence of specialized and costly equipment.
Thus, a need exists for a means that provides for the substantially complete recovery of a cyclic ester such as lactide from a vapor stream that also contains open-chain acids as well as water. A need also exists for such process that also provides for the substantially complete recovery of the acid values as cyclic ester.